Adaptive multi-level threshold system and method for power converter protection

ABSTRACT

System and method for protecting a power converter. The system includes a compensation system configured to receive an input signal and generate a control signal, a cycle threshold generator configured to receive the control signal and generate a cycle threshold, and a comparator configured to receive the cycle threshold and a feedback signal and generate a comparison signal. Additionally, the system includes a pulse-width-modulation generator configured to receive the comparison signal and generate a modulation signal in response to the comparison signal, and a switch configured to receive the modulation signal and control an input current for a power converter. The input current is associated with an output power for the power converter. The cycle threshold corresponds to a threshold power level for the output power. The threshold power level is constant, decreases, or increases with respect to the input signal.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.______ (EastIP Ref. No. 05NI1963-1365-SMY), filed Feb. 3, 2005, entitled“Adaptive Multi-Level Threshold System and Method for Power ConverterProtection,” by Inventors Zhen Zhu, Jun Ye, Shifeng Zhao, Lieyi Fang,and Zhiliang Chen, commonly assigned, incorporated by reference hereinfor all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

Not applicable

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK

Not applicable

BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides an adaptive multi-level thresholdsystem and method for over-current protection. Merely by way of example,the invention has been applied to a power converter. But it would berecognized that the invention has a much broader range of applicability.

Power converters are widely used for consumer electronics such asportable devices. The power converters can convert electric power fromone form to another form. As an example, the electric power istransformed from alternate current (AC) to direct current (DC), from DCto AC, from AC to AC, or from DC to DC. Additionally, the powerconverters can convert the electric power from one voltage level toanother voltage level.

The power converters include linear converters and switch-modeconverters. The switch-mode converters often use pulse-width-modulated(PWM) or pulse-frequency-modulated mechanisms. These mechanisms areusually implemented with a switch-mode controller including variousprotection components. These components can provide over-voltageprotection, over-temperature protection, and over-current protection(OCP). These protections can often prevent the power converters fromsuffering permanent damage.

For example, a conventional OCP uses a single threshold level, which canlimit the current on a cycle-by-cycle or pulse-by-pulse basis. But thisconventional technique usually cannot adequate protect the powerconverters under certain operating conditions.

Hence it is highly desirable to improve techniques for over-currentprotection.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides an adaptive multi-level thresholdsystem and method for over-current protection. Merely by way of example,the invention has been applied to a power converter. But it would berecognized that the invention has a much broader range of applicability.

According to one embodiment of the present invention, a system forprotecting a power converter is provided. The system includes acompensation system configured to receive an input signal and generate acontrol signal, a cycle threshold generator configured to receive thecontrol signal and generate a cycle threshold, and a comparatorconfigured to receive the cycle threshold and a feedback signal andgenerate a comparison signal. Additionally, the system includes apulse-width-modulation generator configured to receive the comparisonsignal and generate a modulation signal in response to the comparisonsignal, and a switch configured to receive the modulation signal andcontrol an input current for a power converter. The input current isassociated with an output power for the power converter. The cyclethreshold corresponds to a threshold power level for the output power.The threshold power level is constant, decreases, or increases withrespect to the input signal.

According to another embodiment of the present invention, a system forprotecting a power converter includes a startup control systemconfigured to generate a first control signal, a cycle thresholdgenerator configured to receive the first control signal and generate acycle threshold, and a comparator configured to receive the cyclethreshold and a feedback signal and generate a comparison signal.Additionally, the system includes a pulse-width-modulation generatorconfigured to receive the comparison signal and generate a modulationsignal in response to the comparison signal, and a switch configured toreceive the modulation signal and control an input current for a powerconverter. The cycle threshold increases with a time during a startupperiod.

According to yet another embodiment of the present invention, a systemfor protecting a power converter includes a cycle threshold generatorconfigured to generate a cycle threshold, a comparator configured toreceive the cycle threshold and a feedback signal and generate acomparison signal, and a pulse-width-modulation generator configured toreceive the comparison signal and generate a modulation signal inresponse to the comparison signal. Additionally, the system includes aswitch configured to receive the modulation signal and control an inputcurrent for a power converter, and a pattern recognition systemconfigured to receive the comparison signal and output a first controlsignal to the cycle threshold generator. The first control signalindicates whether the feedback signal exceeds the cycle threshold at afrequency higher than a predetermined level. The cycle thresholdgenerator is further configured to reduce the cycle threshold from afirst threshold level to a second threshold level if the first controlsignal indicates the feedback signal exceeds the cycle threshold at thefrequency higher than the predetermined level.

According yet another embodiment of the present invention, a system forprotecting a power converter includes a compensation system configuredto receive an input signal and generate a control signal, a thresholdgenerator configured to receive the control signal and generate athreshold, and a comparator configured to receive the threshold and afeedback signal and generate a comparison signal. Additionally, thesystem includes a shutdown control system configured to receive thecomparison signal and generate a shutdown signal, and a switch coupledto the shutdown control system and configured to shut down a powerconverter. The threshold varies with at least the input signal.

According to yet another embodiment of the present invention, a methodfor protecting a power converter includes receiving an input signal,generating a first control signal based on at least informationassociated with the input signal, and generating a second control signalindicating whether a power converter is at a startup state.Additionally, the method includes processing information associated withthe first control signal, the second control signal, and a third controlsignal, generating a cycle threshold based on at least informationassociated with the first control signal, the second control signal, andthe third control signal, and processing information associated with thecycle threshold and a feedback signal. Moreover, the method includesgenerating a comparison signal based on at least information associatedwith the cycle threshold and the feedback signal, processing informationassociated with the comparison signal, and generating the third controlsignal based on at least information associated with the comparisonsignal. The third control signal indicates whether the feedback signalexceeds the cycle threshold at a frequency higher than a predeterminedlevel. Also, the method includes generating a modulation signal based onat least information associated with the comparison signal, anddetermining an input current for the power converter based on at leastinformation associated with the modulation signal.

According to yet another embodiment of the present invention, a methodfor protecting a power converter includes receiving an input signal,generating a control signal based on at least information associatedwith the input signal, and processing information associated with thecontrol signal. Additionally, the method includes generating a thresholdbased on at least information associated with control signal, processinginformation associated with the threshold and a feedback signal, andgenerating a comparison signal based on at least information associatedwith the threshold and the feedback signal. Moreover, the methodincludes processing information associated with the comparison signal,and shutting down the power converter based on at least informationassociated with the comparison signal. The threshold varies with atleast the input signal.

Many benefits are achieved by way of the present invention overconventional techniques. For example, some embodiments of the presentinvention provide multi-level thresholds for over-current protection.For example, the multi-level thresholds correspond to differentoperating conditions. As an example, the operating conditions includeoutput overloading conditions and system startup conditions. In anotherexample, the multi-level thresholds are adaptively adjusted based oninput voltage. In yet another example, the multi-level thresholds are inthe current domain and/or the voltage domain. Certain embodiments of thepresent invention provide a cycle-by-cycle threshold that is compensatedfor variations in input voltage. For example, the maximum output poweris adjusted in response to variations in the input voltage. Someembodiments of the present invention provide a cycle-by-cycle thresholdfor current limiting under normal operations. For example, the maximumoutput power is set to be constant, increasing or decreasing over arange of input voltage under normal operations depending on thecompensation scheme used between over-current threshold level and inputvoltage.

Certain embodiments of the present invention adjust a cycle-by-cyclethreshold in response to a triggering pattern of an over-currentprotection. The triggering pattern reflects the output loadingconditions. For example, if the triggering count is low and randomwithin a given period of time, the output loading condition is usuallynormal. The cycle-by-cycle threshold for normal operations can be used.In another example, if the triggering count exceeds a predeterminedlevel within a given period of time, the cycle-by-cycle threshold isreduced. In yet another example, the maximum output power is reducedunder short-circuit or heavy-overloading conditions until the outputloading condition becomes normal. In yet another example, the powerconverter operates in CCM or DCM. Some embodiments of the presentinvention adjust a cycle-by-cycle threshold to control the currentincrease during system startup. For example, during the startup, thecycle-by-cycle threshold is ramped from a low value to the normal valueused for normal operations. Certain embodiments of the present inventiondistinguish the over-current protection for start-up conditions and theover-current protection for normal operations. During the power supplystartup, the output voltage of a power converter is usually lower thanthe output voltage for normal operations. The feedback loop can forcethe power converter to deliver additional current and thus power to theoutput. Consequently, the transformer winding current can quickly riseto a very high level. This rapid current rise often causes transientsaturation of the winding and damages the converter system due to doublemagnetic flux effect. According to certain embodiments of the presentinvention, the over-current threshold is ramped up during start up inorder to avoid or reduce the transient saturation and double magneticflux effect. Hence the system damage can be reduced or prevented.

Certain embodiments of the present invention provide an abnormalthreshold. For example, the abnormal threshold is higher than, equal to,or lower than the cycle-by-cycle threshold. In another example, theabnormal threshold is used for triggering a system shutdown inshort-circuit and/or over-loading conditions. Some embodiments of thepresent invention provide a self recovery mechanism after apredetermined shutdown period. For example, a power converter canoperate in the “burst mode” to reduce the output power under abnormallyhigh over-current conditions and recover to normal operations once theabnormally high over-current conditions are cleared. Certain embodimentsof the present invention provide an immediate and permanent latchshutdown if the highest over-current threshold is triggered in order toeffectively protect the converter system from damage in a timely manner.Some embodiments of the present invention provide a solution for bothcurrent limiting and system shutdown. Certain embodiments of the presentinvention can shorten the OCP de-bouncing and/or delay and overcome theover-current hole problem. Some embodiments of the present invention canimprove system flexibility, reliability and safety. Certain embodimentsof the present invention can decouple a complicated OCP design into asimple set of protection mechanisms.

Various additional objects, features and advantages of the presentinvention can be more fully appreciated with reference to the detaileddescription and the accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified adaptive multi-level threshold system forover-current protection according to an embodiment of the presentinvention;

FIG. 2 is a simplified adaptive multi-level threshold system forover-current protection according to another embodiment of the presentinvention;

FIG. 3 is a simplified threshold compensation system for over-currentprotection according to an embodiment of the present invention;

FIG. 4 is a simplified diagram showing cycle-by-cycle threshold as afunction of time according to an embodiment of the present invention;

FIG. 5 is a simplified offline flyback converter according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides an adaptive multi-level thresholdsystem and method for over-current protection. Merely by way of example,the invention has been applied to a power converter. But it would berecognized that the invention has a much broader range of applicability.

The conventional over-current protection (OCP) often fails to limit thecurrent or protect the power converter under various conditions. Forexample, the power converter is operated in continuous current mode(CCM). If the output is overloaded or short circuited. The current canstart off at a value higher than the OCP threshold and thus overstressthe power converter. Also, if the power converter experiences inductorsaturation for transformer windings, the OCP control can be delayed forsuch a long period of time that the current may fly off and permanentlydamage the power converter. As another example, the power converter isoperated at low duty cycle. The “on” period is shorter than the OCPcontrol delay, so the OCP fails to respond to the over-currentcondition. In yet another example, the over-current protection isfrequently triggered by certain operating conditions. These conditionscan apply continuous stress to the power converter, and degrade itslong-term reliability. Also, the power converter may fail to meetcertain safety requirements because of high power delivered under theseoperating conditions.

FIG. 1 is a simplified adaptive multi-level threshold system forover-current protection according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. A system 100includes a resistor 102, an input compensation system 110, a startupcontrol system 120, a pattern recognition system 130, a cycle-by-cyclethreshold generator 140, an abnormal threshold generator 150, acycle-by-cycle comparator 160, an abnormal comparator 170, a shutdowncontrol system 180, a pulse-width-modulation (PWM) generator 190, aswitch 192, and a feedback system 194. Although the above has been shownusing a selected group of components for the system 100, there can bemany alternatives, modifications, and variations. For example, some ofthe components may be expanded and/or combined. Other components may beinserted to those noted above. Depending upon the embodiment, thearrangement of components may be interchanged with others replaced. Forexample, the system 100 is used to regulate a power converter. Furtherdetails of these components are found throughout the presentspecification and more particularly below.

The cycle-by-cycle threshold generator 140 receives control signals 142,144, and 146 from the input compensation system 110, the startup controlsystem 120, and the pattern recognition system 130 respectively, andoutputs a cycle-by-cycle threshold signal 148. For example, thecycle-by-cycle threshold signal 148 is used to limit the peak current ofthe power converter. The abnormal threshold generator 150 receives thecontrol signal 142 from the input compensation system 110 and outputs anabnormal threshold signal 152. In one embodiment, the abnormal thresholdgenerator 150 includes n abnormal threshold subsystems, and the abnormalthreshold signal 152 includes values of n abnormal thresholds. n is apositive integer.

The cycle-by-cycle threshold signal 148 represents either acycle-by-cycle threshold voltage or a cycle-by-cycle threshold current.The abnormal threshold signal 152 represents either an abnormalthreshold voltage or an abnormal threshold current. In one embodiment,the cycle-by-cycle threshold voltage is lower than, equal to, or higherthan the abnormal threshold voltage. In another embodiment, thecycle-by-cycle threshold current is lower than, equal to, or higher thanthe abnormal threshold current.

The control signal 142 is generated by the input compensation system110. The input compensation system 110 is connected to the resistor 102,which is also coupled to an input voltage 104. In one embodiment, theinput compensation system 110 senses the input voltage 104 and generatesthe control signal 142. For example, the input compensation system 110receives an input signal, which is proportional to the input voltage.

The control signals 142, 144, and 146 are used by the cycle-by-cyclethreshold generator 140 to determine the cycle-by-cycle threshold signal148. For example, the control signals 142 can adjust the cycle-by-cyclethreshold signal 148 in order to compensate for variations in the inputvoltage 104 during normal operations. In another example, during thestartup of a power converter, the control signal 144 is used to reducethe cycle-by-cycle threshold signal 148 from the value used for normaloperations, and then gradually raise the cycle-by-cycle threshold signal148 to the value for normal operations. In one embodiment, thecycle-by-cycle threshold varies with the time and the input voltage 104during the startup period. In yet another example, the control signal146 can be used to lower the cycle-by-cycle threshold signal 148 fromthe value used for normal operations. In one embodiment, if thefrequency for triggering a cycle-by-cycle over-current protectionexceeds a predetermined threshold within a given period of time, thecontrol signal 146 instructs the cycle-by-cycle threshold generator 140to lower the cycle-by-cycle threshold signal 148. Then the thresholdsignal 148 can recover to the value for normal operations after apredetermined delay. For example, the frequency is equal to a ratio of anumber of triggering to the give period of time. In another example, thelowered cycle-by-cycle threshold signal 148 varies the input voltage104.

The control signal 142 is used by the abnormal threshold generator 140to determine the abnormal threshold signal 152. For example, the controlsignals 142 can adjust the abnormal threshold signal 152 in order tocompensate for variations in the input voltage 104.

The cycle-by-cycle comparator 160 receives the cycle-by-cycle thresholdsignal 148 and a feedback signal 162. The feedback signal 162 indicatesthe magnitude of a current that the system 100 intends to regulate. Forexample, the feedback signal 162 is proportional to an input current ofa power converter. In one embodiment, the cycle-by-cycle thresholdsignal 148 represents a threshold voltage, and the feedback signal 162includes a voltage. In another embodiment, the cycle-by-cycle thresholdsignal 148 represents a threshold current, and the feedback signal 162includes a current. The cycle-by-cycle comparator 160 compares thecycle-by-cycle threshold signal 148 and the feedback signal 162, andgenerates a comparison signal 164.

The abnormal comparator 170 receives the abnormal threshold signal 152and the feedback signal 162. In one embodiment, the abnormal comparator170 includes n abnormal comparator subsystems. Each abnormal comparatorsubsystem receives the feedback signal 162 and information aboutmagnitude of an abnormal threshold from one of the n abnormal thresholdsubsystems. n is a positive integer. For example, the abnormal thresholdsignal 152 represents a threshold voltage, and the feedback signal 162includes a voltage. As another example, the abnormal threshold signal152 represents a threshold current, and the feedback signal 162 includesa current. The abnormal comparator 170 compares the abnormal thresholdsignal 152 and the feedback signal 162, and generates a comparisonsignal 172. In one embodiment, the abnormal comparator 170 includes nabnormal comparator subsystems. Each abnormal comparator subsystemcompares the received abnormal threshold and the feedback signal 162,and generates a component of the comparison signal 172.

The comparison signal 172 is received by the shutdown control system180. If the comparison signal 172 shows that the feedback signal 162exceeds the abnormal threshold signal 152, the shutdown control system180 can generate a shutdown signal 182 after a predetermined delay. Inone embodiment, the shutdown control system 180 includes n shutdowncontrol subsystems. n is a positive integer. Each shutdown controlsubsystem receives a component of the comparison signal 172 from one ofthe n abnormal comparator subsystems. If the component shows that thefeedback signal 162 exceeds the respective abnormal threshold, theshutdown control subsystem can generate the shutdown signal 182 after apredetermined delay. In one embodiment, the predetermined delay can bedifferent or the same between any two shutdown control subsystems. Forexample, the predetermined delay is shorter for a shutdown controlsubsystem that corresponds to a higher threshold.

In another embodiment, the shutdown signal 182 generated by one shutdowncontrol subsystem may be different or the same as the shutdown signal182 generated by another shutdown control subsystem. For example, theshutdown signal 182 specifies the length of a shutdown period when apower converter should remain shutdown. In another example, the shutdownperiod is longer for the shutdown signal 182 generated by a shutdowncontrol subsystem that corresponds to a higher threshold. In yet anotherexample, the shutdown period indicates a permanent shutdown. As anexample, if the highest threshold is exceeded, the shutdown ispermanent.

The pattern recognition system 130 receives the comparison signal 164.In one embodiment, the pattern recognition system uses an up-downcounter scheme or an analog filter approach. The comparison signal 164indicates whether the feedback signal 162 exceeds the cycle-by-cyclethreshold signal 148. If the feedback signal 162 exceeds thecycle-by-cycle threshold signal 148, the cycle-by-cycle over-currentprotection is triggered. In one embodiment, if the triggering frequencyof the cycle-by-cycle over-current protection exceeds a predeterminedthreshold within a given period of time, the control signal 146instructs the cycle-by-cycle threshold generator 140 to lower thecycle-by-cycle threshold signal 148. In another embodiment, the patternrecognition and control is performed repeatedly.

The PWM generator 190 receives the comparison signal 164 and theshutdown signal 182 and generates a modulation signal 196. Themodulation signal 196 is used to turn on or off the switch 192. Forexample, if the comparison signal 164 indicates that the feedback signal162 exceeds the cycle-by-cycle threshold signal 148, the PWM generator190 activates the cycle-by-cycle over-current protection and changes themodulation signal 196. The changed modulation signal 196 turns off theswitch 192. In another example, if the shutdown signal 182 includes ashutdown command with a shutdown period, the modulation signal 196 isgenerated to turn off the switch 192. After the shutdown period, theswitch 192 is again turned on by the modulation signal 196. For example,the power converter is restarted, and the startup control 120 isactivated.

The switch 192 provides certain control over the current that the system100 intends to regulate. For example, the switch 192 includes atransistor, whose gate is connected to the modulation signal 196. In oneembodiment, the transistor is a MOSFET.

The feedback system 194 generates the feedback signal 162. For example,the feedback system 194 includes a current sensing device. The feedbacksystem 194 may or may not convert the sensed current into a voltage. Inanother example, the feedback signal 162 indicates the magnitude of acurrent that the system 100 intends to regulate. In one embodiment, thefeedback signal 162 represents the magnitude with voltage. In anotherembodiment, the feedback signal 162 represents the magnitude withcurrent.

As discussed above, in one embodiment, the control signals 142 canadjust the cycle-by-cycle threshold signal 148 in order to compensatefor variations in the input voltage 104 during normal operations. Thecycle-by-cycle threshold signal 148 is used to regulate an input currentof a power converter. The power converter can convert the input voltageinto an output voltage associated with an output power of the powerconverter. The cycle-by-cycle threshold signal 148 corresponds to athreshold input current, and a threshold power level for the outputpower. The threshold power level is a function of the input voltage 104.For example, the threshold power level remains constant with respect tothe input voltage 104. In another example, the threshold power levelincreases or decreases with the input voltage 104. In yet anotherexample, the threshold power level is used as the maximum output powerallowable by the cycle-by-cycle threshold.

FIG. 2 is a simplified adaptive multi-level threshold system forover-current protection according to another embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. A system 200includes transistors 201 through 217, resistors 220 and 222, operationsamplifiers 230 and 232, a voltage divider 234, a multiplexer 240, alogic system 242, a pattern recognition system 250, comparators 260, 262and 264, a controller 270, and a switch 272. Although the above has beenshown using a selected group of components for the system 200, there canbe many alternatives, modifications, and variations. For example, someof the components may be expanded and/or combined. Other components maybe inserted to those noted above. Depending upon the embodiment, thearrangement of components may be interchanged with others replaced. Forexample, the system 200 is an example of the system 100. Further detailsof these components are found throughout the present specification andmore particularly below.

The transistors 201 through 208 are used to sense an input voltage 302.In one embodiment, the transistors 201 and 202 form a current mirror.For example, the transistors 201 and 202 are PMOS transistors. The drainof the transistor 201 is connected to a supply voltage 304. The sourcesof the transistors 201 and 202 are both connected to the input voltage302 through the resistor 220. For example, the resistor 220 has aresistance ranging from several hundred kΩ to several MΩ. Through theresistor 220, a current 306 is distributed between the transistors 201and 202 based on the sizes of these two transistors. Additionally, thecurrent 306 is proportional to the input voltage 302.

The transistors 203 and 204 form a cascode. In one embodiment, thecascode enhances the output impendence of the current mirror includingthe transistors 201 and 202, and improves the accuracy of the sensedcurrent. In another embodiment, the transistors 203 and 204 are PMOStransistors. The transistors 205 and 206 form a pair of switches. In oneembodiment, the switches are used to provide additional control forsensing the input voltage 302. In another embodiment, the transistors205 and 206 are PMOS transistors.

The transistors 207 and 208 form a current mirror. In one embodiment,the transistor 207 provides a drain current and a bias to the cascodeincluding the transistors 203 and 204. The transistor 208 sinks andmirrors the sensed current to the transistor 207 and other componentssuch as the transistor 209. The sensed current indicates the magnitudeof the input voltage 302. For example, the sensed current isproportional to the input voltage 302. In another embodiment, thetransistors 207 and 208 are NMOS transistors.

The transistors 209 through 214 are used for summing currents. In oneembodiment, the transistors 209 through 212 are NMOS transistors, andthe transistors 213 and 214 are PMOS transistors. In another embodiment,the sensed current is mirrored by the transistor 209. A referencecurrent 308 is fed into the transistor 210 and mirrored by thetransistors 211 and 212. A drain current 312 of the transistor 212 isfed into the negative input terminal of the operational amplifier 230.The drain current 312 is equal to the reference current 308. Thereference current 308 and a current 310 equal to the sensed current areadded up at the drain of the transistor 213. A drain current of thetransistor 213 is mirrored by the transistor 214, which outputs acurrent 314 to the positive input terminal of the operations amplifier232. The current 314 equals the sum of the reference current 308 and thecurrent 310.

The operational amplifiers 230 and 232, the transistors 215 and 216, andthe voltage divider 234 are used to adjust the cycle-by-cycle thresholdand the abnormal threshold and compensate for variations in the inputvoltage 302. FIG. 3 is a simplified threshold compensation system forover-current protection according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications.

As shown in FIGS. 2 and 3, an output terminal 402 of the operationalamplifier 230 is connected to the voltage divider 234. In oneembodiment, the voltage divider 234 includes a plurality of resistors inseries. For example, the plurality of resistors includes a resistor 404with two terminals 406 and 408. The terminal 406 is connected to theterminal 402. The terminal 408 is connected to the negative inputterminal of the operational amplifier 230 through the transistors 215and 216. For example, the transistors 215 and 216 are two identical NMOStransistors. The two transistors 215 and 216, the operational amplifier230, and the resistor 404 form a loop. The drains of the transistors 215and 216 are connected to the positive input terminal of the operationalamplifier 232. The gates of the transistors 215 and 216 are connected toan output terminal 410 of the operational amplifier 232. The operationalamplifier 232 and the transistors 215 and 216 form another loop.

The positive input terminal of the operational amplifier 230 and thenegative input terminal of the operational amplifier 232 are connectedto two reference voltages 412 and 414 respectively. For example, thereference voltage 412 is represented by V_(bias1), and the referencevoltage 414 is represented by V_(bias2). A drain terminal 416 of thetransistors 215 and 216 has a voltage level substantially equal toV_(bias2), and the negative input terminal of the operational amplifier230 has a voltage level substantially equal to V_(bias1).

The current 312 is equal to the reference current 308 and flows from thenegative input terminal of the operational amplifier 230 and the sourceof the transistor 215. For example, the magnitude of the current 312 isrepresented by I_(ref). Additionally, the current 314 is equal to thesum of the reference current 308 and the current 310. For example, themagnitude of the current 310 is represented by I_(sense), and themagnitude of the current 314 is represented by I_(ref)+I_(sense). Thecurrent 314 flows to the positive input terminal of the operationalamplifier 232 and the drain terminal 416 of the transistors 215 and 216.

When the currents reach balance, the drain currents of the transistors215 and the 216 are substantially equal to I_(ref) and I_(sense)respectively. A current source 418 is connected to the source of thetransistor 216, and used to sink the drain current of the transistor216. For example, the current source 418 includes the transistor 217.

In one embodiment, if the transistor 215 operates in triode region, thedrain current of the transistor 215 can be calculated as follows:$\begin{matrix}{I_{d\quad 215} = {\frac{\mu\quad C_{ox}}{2}{\frac{W}{L}\lbrack {{2( {V_{gs} - V_{t}} )V_{{ds}\quad 215}} - V_{{ds}\quad 215}^{2}} \rbrack}}} & ( {{Equation}\quad 1} )\end{matrix}$

where I_(d215) represents the drain current of the transistor 215.V_(ds215) is the drain voltage with respect to the source voltage forthe transistor 215, and V_(gs) is the gate voltage with respect to thesource voltage for the transistor 215. V_(t) is the threshold voltage ofthe transistor 215. W and L are the width and length of the transistor215. μ is the electron mobility of the transistor 215, and C_(ox) is theunit-area capacitance across the gate oxide of the transistor 215.

If V_(gs)−V_(t)>>V_(ds215), Equation 1 can be simplified as follows:$\begin{matrix}{I_{d} \approx {\frac{W}{L}\mu\quad{C_{ox}( {V_{gs} - V_{t}} )}V_{{ds}\quad 215}}} & ( {{Equation}\quad 2} )\end{matrix}$

Therefore, $\begin{matrix}{V_{{ds}\quad 216} = {{V_{{ds}\quad 215}\frac{I_{{ds}\quad 216}}{I_{{ds}\quad 215}}} = \frac{L \cdot I_{sense}}{W\quad\mu\quad{C_{ox}( {V_{gs} - V_{T}} )}}}} & ( {{Equation}\quad 3} )\end{matrix}$

where I_(d216) represents the drain current of the transistor 216, andV_(ds216) represents the drain voltage with respect to the sourcevoltage for the transistor 216. Accordingly, the voltage level at theterminal 408 is determined as follows: $\begin{matrix}{V_{terminal} = {{V_{{bias}\quad 2} - V_{{ds}\quad 216}} = {V_{{bias}\quad 2} - \frac{L \cdot I_{sense}}{W\quad\mu\quad{C_{ox}( {V_{gs} - V_{T}} )}}}}} & ( {{Equation}\quad 4} )\end{matrix}$

where V_(terminal) represents the voltage level at the terminal 408. Thesensed current I_(sense) indicates the magnitude of the input voltage302. For example, the sensed current is proportional to the inputvoltage 302. According to Equation 4, V_(terminal) is adjusted tocompensate for variations in the input voltage 302.

In another embodiment, the voltage divider 234 includes n resistors aswell as the resistor 404. n is a positive integer. The n resistorsprovide n voltage levels to the multiplexer 240. For example, the nvoltage levels include V_(thn), V_(th(n−1)), . . . , V_(th2), andV_(th1). V_(thn) is equal to V_(terminal).

An abnormal threshold voltage 340 of the system 200 equals the voltagelevel at the terminal 406. A cycle-by-cycle threshold voltage 350 of thesystem 200 is generated by the multiplexer 240 controlled by a logicsystem 242. In one embodiment, the cycle-by-cycle threshold voltage 350for normal operations is determined based on the n voltage levelsaccording to a predetermined logic. For example, the cycle-by-cyclethreshold voltage 350 equals V_(thn) for normal operations. In anotherembodiment, during the startup of a power converter, the cycle-by-cyclethreshold voltage 350 is increased over time. For example, thecycle-by-cycle threshold voltage 350 increases stepwise from V_(th1),V_(th2), . . . , V_(th(n−1)), and V_(thn) according to instructions fromthe control system 242. In another example, the logic system 242 is usedas the startup control system 120.

In yet another embodiment, the cycle-by-cycle threshold voltage 350 islowered by the multiplexer 240 in response to a signal 316 from thepattern recognition system 250. The pattern recognition detects thetriggering frequency of a cycle-by-cycle over-current protection. If thefrequency exceeds a predetermined threshold within a given period oftime, the signal 316 instructs the multiplexer 240 to lower thecycle-by-cycle threshold voltage 350 according to a pre-determinedlogic. After a predetermined delay, the cycle-by-cycle threshold voltage350 can recover to the value for normal operations. For example, thecycle-by-cycle threshold voltage 350 decreases from one of V_(th1),V_(th2), . . . , V_(th(n−1)) and V_(thn), to another of V_(th1),V_(th2), . . . , V_(th(n−1)) and V_(thn) according to instructions fromthe pattern recognition system 250.

The pattern recognition system 250, for example, includes a low passfilter 252, a comparator 254, and a timer 256. The output signal 316 isused to adaptively adjust the threshold voltage 350.

The cycle-by-cycle threshold voltage 350 and the abnormal thresholdvoltage 340 are used for triggering an over-current protection. Theabnormal threshold voltage 340 is provided to the abnormal comparator260. The abnormal comparator 260 also receives a sensed voltage 318. Thesensed voltage 318 is proportional to a current 320 that the system 200intends to regulate. For example, the sensed voltage 318 is generated bythe resistor 222. The abnormal comparator 260 compares the abnormalthreshold voltage 340 and the sensed voltage 318, and generates acomparison signal 322.

The cycle-by-cycle threshold voltage 350 is provided to thecycle-by-cycle comparator 262, which also receives the sensed voltage318. The cycle-by-cycle comparator 262 compares the cycle-by-cyclethreshold voltage 350 and the sensed voltage 318, and generates acomparison signal 324. The comparison signal 324 is sent to both thecontroller 270 and the pattern recognition system 250.

Additionally, the sensed voltage 318 is provided to the PWM comparator264, which also receives a feedback (FB) voltage 326 generated by afeedback loop. The PWM comparator 264 compares the feedback voltage 326and the sensed voltage 318 and generates an output signal 328.

The controller 270 receives the signals 322, 324, and 328, and generatesa modulation signal 330. The modulation signal 330 is used to turn on oroff the switch 272. For example, the switch 272 includes a transistor.For example, if the signal 322 indicates that the sensed voltage 318exceeds the cycle-by-cycle threshold voltage 350, the controller 270activates the cycle-by-cycle over-current protection and changes themodulation signal 330. The changed modulation signal 330 is used toturns off the switch 272. In another example, if the signal 324indicates that the sensed voltage 318 exceeds the abnormal thresholdvoltage 340, the modulation signal 330 is generated to turn off theswitch 272 either immediately or after a predetermined delay. Theshutdown is either permanent or temporary. For example, after apredetermined shutdown period, the switch 272 is again turned on by themodulation signal 330. In yet another example, if the signal 328indicates that the sensed voltage 318 exceeds the feedback voltage 326,the modulation signal 330 is generated to turn off the switch 272.

As discussed above, in one embodiment, the cycle-by-cycle thresholdvoltage 350 is compensated for variations in the input voltage 302during normal operations. The cycle-by-cycle threshold voltage 350 isused to regulate the current 320 of a power converter. The powerconverter can convert the input voltage 302 into an output voltageassociated with an output power of the power converter. Thecycle-by-cycle threshold voltage 350 corresponds to a threshold currentlevel for the current 320, and a threshold power level for the outputpower. The threshold power level is a function of the input voltage 302.For example, the threshold power level remains constant with respect tothe input voltage 302. In another example, the threshold power levelincreases or decreases with the input voltage 302. In yet anotherexample, the threshold power level is used as the maximum output powerallowable by the cycle-by-cycle threshold voltage 350. In yet anotherexample, the transistors 208 and 209 form a current mirror. The gain ofthe current mirror can determine whether the threshold power level isconstant, increases, or decreases with respect to the input voltage 302.In one embodiment, the gain of the current mirror is adjusted in orderto select whether the threshold power level is constant, increases, ordecreases with respect to the input voltage 302.

FIG. 4 is a simplified diagram showing cycle-by-cycle threshold as afunction of time according to an embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. A vertical axis 510represents cycle-by-cycle threshold voltage or current, and a horizontalaxis 520 represents time. A curve 530 shows that the cycle-by-cyclethreshold equals to a starting value 532 at the beginning of the startupand then increases over time during the startup. The curve 530 may beproduced by the system 100 and/or the system 200.

As discussed above and further emphasized here, FIGS. 1-4 are merelyexamples, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the cycle-by-cycle threshold generator140 is replaced by another type of cycle threshold generator. In oneembodiment, the cycle threshold generator includes an n-cycle-by-n-cyclethreshold generator. n is a positive integer. In another example, thecycle-by-cycle threshold signal 148 is replaced by another type of cyclethreshold signal. In one embodiment, the cycle threshold signal includesan n-cycle-by-n-cycle threshold signal. n is a positive integer. In yetanother example, the cycle-by-cycle threshold represented by thecycle-by-cycle threshold signal 148 is replaced by another type of cyclethreshold. In one embodiment, the cycle threshold includes ann-cycle-by-n-cycle threshold. n is a positive integer. In yet anotherexample, the cycle-by-cycle threshold voltage 350 is replaced by anothertype of cycle threshold voltage. In one embodiment, the cycle thresholdvoltage includes an n-cycle-by-n-cycle threshold voltage. n is apositive integer.

According to yet another embodiment of the present invention, a methodfor protecting a power converter includes receiving an input signal,generating a first control signal based on at least informationassociated with the input signal, and generating a second control signalindicating whether a power converter is at a startup state.Additionally, the method includes processing information associated withthe first control signal, the second control signal, and a third controlsignal, generating a cycle threshold based on at least informationassociated with the first control signal, the second control signal, andthe third control signal, and processing information associated with thecycle threshold and a feedback signal. Moreover, the method includesgenerating a comparison signal based on at least information associatedwith the cycle threshold and the feedback signal, processing informationassociated with the comparison signal, and generating the third controlsignal based on at least information associated with the comparisonsignal. The third control signal indicates whether the feedback signalexceeds the cycle threshold at a frequency higher than a predeterminedlevel. Also, the method includes generating a modulation signal based onat least information associated with the comparison signal, anddetermining an input current for the power converter based on at leastinformation associated with the modulation signal. For example, themethod can be performed by the system 100 and/or the system 200.

According to yet another embodiment of the present invention, a methodfor protecting a power converter includes receiving an input signal,generating a control signal based on at least information associatedwith the input signal, and processing information associated with thecontrol signal. Additionally, the method includes generating a thresholdbased on at least information associated with control signal, processinginformation associated with the threshold and a feedback signal, andgenerating a comparison signal based on at least information associatedwith the threshold and the feedback signal. Moreover, the methodincludes processing information associated with the comparison signal,and shutting down the power converter based on at least informationassociated with the comparison signal. The threshold varies with atleast the input signal. For example, the method can be performed by thesystem 100 and/or the system 200.

The present invention has various applications. For example, the system100 and/or the system 200 can provide an over-current protection (OCP)scheme to a switch-mode power converter with pulse width modulation. Forexample, the switch-mode power converter is a forward converter or anoffline fly-back converter. In another example, the switch-mode powerconverter operates in discontinuous current mode (DCM) or continuouscurrent mode (CCM).

FIG. 5 is a simplified offline flyback converter according to anembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. The system 500 includes a PWM controller 510. The PWMcontroller 510 includes a PWM generator 520 coupled to a plurality ofprotection systems. The plurality of protection systems includes asystem 522 for over-temperature protection (OTP), a system 524 forover-voltage protection (OVP), a system 526 for under-voltage lockout(UVLO), and a system 528 for over-current protection (OCP). For example,the OCP system 528 includes some or all components of the system 100. Inanother example, the OCP system 528 includes some or all components ofthe system. The PWM controller 510 receives a sensed signal 530 andoutputs a control signal 532. The sensed signal 530 indicates themagnitude of a current 534 in either the voltage domain or the currentdomain. The control signal 532 is used to turn on or off a switch 540.The switch 540 can control the current flowing through a primary winding550 of the system 500.

As shown in FIG. 5, the OCP system 528 can limit the current 534 fromexceeding a preset value. Without a proper OCP protection scheme, thecurrent 534 can reach a high level when the output short circuit or overloading occurs. This high current can cause damage to the switch 540 dueto excessive voltage stress resulting from L×di/dt at switching orthermal run-away at operation. L represents the inductance of theprimary winding 550, and i represents the current flowing through theprimary winding 550. Additionally, without a proper OCP protectionscheme, the rectifier components at the secondary side of the powerconverter 500 can suffer permanent damage due to excessive voltage andcurrent stress level at switching in the power converter 500.Accordingly, the OCP system 528 according to certain embodiments of thepresent invention can provide important protection for a reliable switchmode converter design.

The present invention has various advantages. Some embodiments of thepresent invention provide multi-level thresholds for over-currentprotection. For example, the multi-level thresholds correspond todifferent operating conditions. As an example, the operating conditionsinclude output overloading conditions and system startup conditions. Inanother example, the multi-level thresholds are adaptively adjustedbased on input voltage. In yet another example, the multi-levelthresholds are in the current domain and/or the voltage domain. Certainembodiments of the present invention provide a cycle-by-cycle thresholdthat is compensated for variations in input voltage. For example, themaximum output power is adjusted in response to variations in the inputvoltage. Some embodiments of the present invention provide acycle-by-cycle threshold for current limiting under normal operations.For example, the maximum output power is set to be constant, increasingor decreasing over a range of input voltage under normal operationsdepending on the compensation scheme used between over-current thresholdlevel and input voltage.

Certain embodiments of the present invention adjust a cycle-by-cyclethreshold in response to a triggering pattern of an over-currentprotection. The triggering pattern reflects the output loadingconditions. For example, if the triggering count is low and randomwithin a given period of time, the output loading condition is usuallynormal. The cycle-by-cycle threshold for normal operations can be used.In another example, if the triggering count exceeds a predeterminedlevel within a given period of time, the cycle-by-cycle threshold isreduced. In yet another example, the maximum output power is reducedunder short-circuit or heavy-overloading conditions until the outputloading condition becomes normal. In yet another example, the powerconverter operates in CCM or DCM. Some embodiments of the presentinvention adjust a cycle-by-cycle threshold to control the currentincrease during system startup. For example, during the startup, thecycle-by-cycle threshold is ramped from a low value to the normal valueused for normal operations. Certain embodiments of the present inventiondistinguish the over-current protection for start-up conditions and theover-current protection for normal operations. During the power supplystartup, the output voltage of a power converter is usually lower thanthe output voltage for normal operations. The feedback loop can forcethe power converter to deliver additional current and thus power to theoutput. Consequently, the transformer winding current can quickly riseto a very high level. This rapid current rise often causes transientsaturation of the winding and damages the converter system due to doublemagnetic flux effect. According to certain embodiments of the presentinvention, the over-current threshold is ramped up during start up inorder to avoid or reduce the transient saturation and double magneticflux effect. Hence the system damage can be reduced or prevented.

Certain embodiments of the present invention provide an abnormalthreshold. For example, the abnormal threshold is higher than, equal to,or lower than the cycle-by-cycle threshold. In another example, theabnormal threshold is used for triggering a system shutdown inshort-circuit and/or over-loading conditions. Some embodiments of thepresent invention provide a self recovery mechanism after apredetermined shutdown period. For example, a power converter canoperate in the “burst mode” to reduce the output power under abnormallyhigh over-current conditions and recover to normal operations once theabnormally high over-current conditions are cleared. Certain embodimentsof the present invention provide an immediate and permanent latchshutdown if the highest over-current threshold is triggered in order toeffectively protect the converter system from damage in a timely manner.Some embodiments of the present invention provide a solution for bothcurrent limiting and system shutdown. Certain embodiments of the presentinvention can shorten the OCP de-bouncing and/or delay and overcome theover-current hole problem. Some embodiments of the present invention canimprove system flexibility, reliability and safety. Certain embodimentsof the present invention can decouple a complicated OCP design into asimple set of protection mechanisms.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

1. A system for protecting a power converter, the system comprising: acompensation system configured to receive an input signal and generate acontrol signal; a cycle threshold generator configured to receive thecontrol signal and generate a cycle threshold; a comparator configuredto receive the cycle threshold and a feedback signal and generate acomparison signal; a pulse-width-modulation generator configured toreceive the comparison signal and generate a modulation signal inresponse to the comparison signal; a switch configured to receive themodulation signal and control an input current for a power converter;wherein: the input current is associated with an output power for thepower converter; the cycle threshold corresponds to a threshold powerlevel for the output power; the threshold power level is constant,decreases, or increases with respect to the input signal.
 2. The systemof claim 1 wherein: the input signal is proportional to an inputvoltage; the power converter is configured to convert the input voltageto an output voltage; the output voltage is associated with the outputpower.
 3. The system of claim 2 wherein the threshold power level isconstant with the input voltage.
 4. The system of claim 2 wherein thethreshold power level decreases with the input voltage.
 5. The system ofclaim 2 wherein the threshold power level increases with the inputvoltage.
 6. The system of claim 2 wherein: the feedback signal isproportional to the input current; the power converter is configured toconvert the input voltage to an output voltage; the output voltage isassociated with the output power.
 7. The system of claim 1 wherein thecycle threshold includes at least one selected from a group consistingof a threshold voltage or a threshold current.
 8. A system forprotecting a power converter, the system comprising: a startup controlsystem configured to generate a first control signal; a cycle thresholdgenerator configured to receive the first control signal and generate acycle threshold; a comparator configured to receive the cycle thresholdand a feedback signal and generate a comparison signal; apulse-width-modulation generator configured to receive the comparisonsignal and generate a modulation signal in response to the comparisonsignal; a switch configured to receive the modulation signal and controlan input current for a power converter; wherein the cycle thresholdincreases with a time during a startup period.
 9. The system of claim 8,and further comprising: a compensation system configured to receive aninput signal and generate a second control signal; wherein: the cyclethreshold generator is further configured to receive the second controlsignal and generate the cycle threshold based on at least the firstcontrol signal and the second control signal; the cycle threshold varieswith the time and the input signal during the startup period.
 10. Thesystem of claim 8 wherein the feedback signal is proportional to theinput current.
 11. The system of claim 8 wherein the cycle thresholdgenerator comprises a multiplexer configured to receive a plurality ofthreshold levels and the first control signal and generate the cyclethreshold based on at least information associated with the plurality ofthreshold levels and the first control signal.
 12. The system of claim10 wherein: the multiplexer is further configured to select a firstthreshold level from the plurality of threshold levels at a first timeand a second threshold level from the plurality of threshold levels at asecond time; the second time is later than the first time; the secondthreshold level is higher than the first threshold level.
 13. A systemfor protecting a power converter, the system comprising: a cyclethreshold generator configured to generate a cycle threshold; acomparator configured to receive the cycle threshold and a feedbacksignal and generate a comparison signal; a pulse-width-modulationgenerator configured to receive the comparison signal and generate amodulation signal in response to the comparison signal; a switchconfigured to receive the modulation signal and control an input currentfor a power converter; a pattern recognition system configured toreceive the comparison signal and output a first control signal to thecycle threshold generator; wherein: the first control signal indicateswhether the feedback signal exceeds the cycle threshold at a frequencyhigher than a predetermined level; the cycle threshold generator isfurther configured to reduce the cycle threshold from a first thresholdlevel to a second threshold level if the first control signal indicatesthe feedback signal exceeds the cycle threshold at the frequency higherthan the predetermined level.
 14. The system of claim 13, and furthercomprising: a compensation system configured to receive an input signaland generate a second control signal; wherein: the cycle thresholdgenerator is further configured to receive the second control signal andgenerate the cycle threshold based on at least the first control signaland the second control signal; the cycle threshold varies with at leastthe input signal.
 15. The system of claim 13 wherein the feedback signalis proportional to the input current.
 16. The system of claim 13 whereinthe cycle threshold generator is further configured to change the cyclethreshold from the second threshold level to a third threshold levelafter a predetermined period of time.
 17. The system of claim 16 whereinthe third threshold level is equal to the first threshold level.
 18. Thesystem of claim 13 wherein the cycle threshold generator comprises amultiplexer configured to receive a plurality of threshold levels andthe first control signal and generate the cycle threshold based on atleast information associated with the plurality of threshold levels andthe first control signal.
 19. The system of claim 18 wherein themultiplexer is further configured to select the first threshold levelfrom the plurality of threshold levels and the second threshold levelfrom the plurality of threshold levels. 20.-33. (canceled)